A capacitor generally includes two electrical conductors in close proximity to, but separated from, each other. The two conductors form the “plates” of the capacitor, and may be separated by a dielectric material. When a voltage is applied across the plates of a capacitor, electrical charge accumulates on the plates. If the plates are electrically isolated essentially immediately after a voltage is applied, the accumulated charge may be stored on the plates, thus “storing” the applied voltage difference.
The fabrication of integrated circuits involves the formation of conductive layers for use as various circuit components, including for use as capacitor plates. Memory circuits, such as DRAMs the like, use conductive layers to form the opposing plates of storage cell capacitors.
The drive for higher-performance, lower-cost integrated circuits dictates ever-decreasing area for individual circuit features, including storage capacitors. Since capacitance of a capacitor (the amount of charge that can be stored as a function of applied voltage) generally varies with the area of capacitor plates, as the circuit area occupied by the storage capacitor decreases, it is desirable to take steps to preserve or increase capacitance despite the smaller occupied area, so that circuit function is not compromised.
Various steps may be taken to increase or preserve capacitance without increasing the occupied area. For example, material(s) having higher dielectric constant may be used between the capacitor plates. Further, the plate surfaces may be roughened to increase the effective surface area of the plates without increasing the area occupied by he capacitor.
One method for providing a roughened surface for a plate of a storage cell capacitor is to form the plate of hemispherical grain polysilicon (HSG), possibly with an overlying metal layer. The hemispherical grains of HSG enhance the surface area of the plate without increasing its occupied area.
HSG presents difficulties in fabrication, however, because of the formation of silicon dioxide on and near the HSG. A silicon dioxide layer may form on the HSG, particularly during deposition of the capacitor's dielectric layer. Even with an intervening metal layer present, oxygen from the deposition of the dielectric layer can diffuse through the metal layer, forming silicon dioxide at the polysilicon surface. Silicon diffusion through the metal layer may also produce a silicon dioxide layer between the metal and the dielectric layers.
Silicon dioxide between the metal layer and the HSG can degrade the electrical contact to the metal capacitor plate surface. Silicon dioxide between the metal layer and the dielectric layer can decrease the capacitance of the resulting capacitor.
To attempt to avoid these negative effects caused by formation of silicon dioxide, a diffusion barrier layer may be employed between the HSG and the metal layer. But in the typical capacitor geometry, the greater the total number of layers, the larger the required minimum area occupied by the capacitor. Further, the upper surface of each additional layer deposited over the HSG tends to be smoother than the underlying surface, reducing the increased surface area provided by the HSG.